tion

$00$

$.$

$$As we've indicated, the switch reading the value and storing it in memory location memory

$[$

operand

$]$

$.$

$$The value is then

read into location

$09.$

At this point, simulation of the first instruction is completed. All that remains is to prepare

the Simpletron to execute the next instruction. Since the instruction just performed was not a

transfer of control, we need merely increment the instruction

$-$

counter register as follows:

instructionCounter

$+$

$+$

;

This action completes the simulated execution of the first instruction. The entire process

$($

i

$.$

e

$.$

$,$

$$the

instruction

$-$

execution cycle

$)$

$$begins anew with the fetch of the next instruction to execute.

Now let's consider how the branching instructions

$-$

$$the transfers of control

$-$

are simulated.

All we need to do is adjust the value in the instruction counter appropriately. Therefore, the

unconditional branch instruction

$($

$40$

$)$

$$is simulated within the switch as

instructionCounter

$=$

$$operand;

The conditional "branch if accumulator is zero" instruction is simulated as

if

$($

accumulator

$=$

$=$

$0$

$)$

$\{$

instructionCounter

$=$

$$operand;

$\}$

At this point, you should implement your Simpletron simulator and run each of the SML

programs you wrote in Exercise

$7\mathrm{.}36$

$.$

$$If you desire, you may embellish SML with additional fea

$-$

tures and provide for these features in your simulator.

Your simulator should check for various types of errors. During the program

$-$

loading phase,

for example, each number the user types into the Simpletron's memory must be in the range

$-$

$9999$

to

$+$

$9999$

$.$

$$Your simulator should test that each number entered is in this range and, if not, keep

prompting the user to re

$-$

enter the number until the user enters a correct number.

During the execution phase, your simulator should check for various serious errors, such as

attempts to divide by zero, attempts to execute invalid operation codes, and accumulator overflows

$($

i

$.$

e

$.$

$,$

$$arithmetic operations resulting in values larger than

$+$

$9999$

$$or smaller than

$-$

$9999$

$)$

$.$

$$Such seri

$-$

ous errors are called fatal errors. When a fatal error is detected, your simulator should display an

error message, such as

$*$

$*$

$*$

$*$

$*$

$*$

$$Attempt to divide by zero

$*$

$*$

$*$

$*$

$*$

$*$

$*$

$*$

$*$

$$Simpletron execution abnormally terminated

$*$

$*$

$*$

$*$

$*$

$*$

and should display a full computer dump in the format we discussed previously. This treatment

will help the user locate the error in the program.